ABSTRACT Stereotactic radiosurgery (SRS) is a non-surgical technique used to treat functional abnormalities and small tumors of the brain. It delivers precisely targeted radiation in fewer high dose treatments than traditional therapy and allows access to sites that would otherwise be difficult or inadvisable to treat due to potential surgical complications to nearby nerves, arteries, and other vital structures. To achieve the 1-2mm precision for intracranial SRS, a metal head ring is rigidly fixated to the patient?s skull using screws under local anesthesia, and then bolted to the treatment couch. The discomfort, inconvenience, and invasive nature associated with the frame preparation have been identified as a serious cause of poor patient compliance and poor clinical efficiencies when SRS is medically indicated. For certain patients, with extreme cranial anatomy or prior surgical bone flaps, ring placement is not possible. In addition, the frame prohibits cases when a hypo- fractionated scheduled is desired leading to the use of techniques with far less accuracy. For clinics, with tight patient linear accelerator (LINAC) scheduling, or high patient to LINAC volumes, frame based SRS scheduling can prove to be problematic due to the necessity of performing the CT simulation, treatment planning, LINAC SRS QA, patient setup, and treatment on the same day. Research aimed at eliminating the frame through the use of thermoplastic face masks have resulted in SRS with less accuracy as mask flex can lead to systematic drift of up to 2-3mm away from the intended target due to rotation about the fulcrum at the back of the skull. Additionally, mask based immobilization accuracy is highly dependent on mask manufacturing quality, skill of the person applying the mask, shrinkage of the mask during treatment, and physical changes of the patient?s head due to swelling or weight loss. In certain cases this has led to uncertainties as large as 6 mm and 2.5 degrees. Such accuracies are not suitable for deep tumors located near critical structures such as the brain stem or for newer treatment modalities such as single iso-center multiple target SRS which are highly sensitive to rotational errors. We propose to solve these problems by developing a novel robotic SRS system that does not require a frame or mask. The hypothesis is that the use of real-time 6 degree of freedom (6DOF) patient head motion tracking and active robotic control systems can assist patients in maintaining a stable sub-millimeter sub-degree head position for long periods of time. Specific aims include: (1) To develop an advanced real-time 6DOF trajectory control law. (2) Design and construction of a clinical robotic SRS system using real-time 3D surface image tracking. (3) Anthropomorphic phantom, healthy volunteer and patient clinical trials.